The present invention is directed to optical communication systems and in particular to the optical isolators that they use.
The American National Standards Institute (ANSI) has developed two generations of major specifications for local-area network (LANs) and is currently developing the third generation. Ethernet can be viewed as the first major LAN specification. It was intended primarily for LANs that use coaxialcable (RG-8) transmission media and operate at 10 Mb/sec., although the basic Ethernet protocol has also been employed with other types of transmission media. The Ethernet specification was completed in 1982. The second major LAN specification was the Fiber Distribution Data Interface (FDDI) specification. FDDI, completed in 1990, is intended for LANs that operate at 100 Mb/sec. on multi-mode fiber with non-physical-contact connectors and use standard (non-lasing) light-emitting diodes (LEDs), although proposals have been made to use the FDDI protocol with other types of media and signal sources, too. The specifics of the third generation are not yet clear, but it will be intended for LANs that operate at 1,000 Mb/sec. (1 Gb/sec.), use laser diodes, and employ single-mode fiber with physical-contact connectors.
This progression of signal sources and transmission media results in large part from cost considerations. LEDs and non-physical-contact connectors are adequate for FDDI-rate transmission and are much less expensive than laser diodes and physical-contact connectors. They are thus the source and connection type of choice for FDDI LANs, and this choice dictates the use of multi-mode fibers; non-lasing diodes do not produce the spatial coherence necessary to couple light efficiently into single-mode fibers.
But the initial choice of lower-cost, standard-LED transmission and non-physical-contract connectors for an FDDI LAN presents the user with a problem if he wants the flexibility to expand to the next, 1-Gb/sec. standard at some point: will he have to bear the expense of "re-wiring" with physical-contact connectors and single-mode fiber in order to upgrade? If so, the apparent cost benefits of the standard-LED, non-physical-contact FDDI system may be illusory.
Answering this re-wiring question involves considering the reasons for using single-mode fibers and physical-contact connectors for high data rates. The reason for using single-mode fibers for high data rates is that multi-mode fibers suffer from intermodal dispersion, which can degrade the optical signal to an extent that is unacceptable for high data rates. Fortunately, the extent of the degradation depends on the length of the cable, so the intermodal dispersion associated with multi-mode fibers can be tolerated even at very high data rates if the cable is only as long as those employed in many local-area networks. If his cable runs are short, therefore, the user avoids the need to employ multi-mode cable.
But physical-contact connectors are necessary at high data rates even if the cable is short: as a practical matter, transmission in the 1-Gb/sec. range requires the use of lasers, and laser coherence can suffer from the effects of light reflected back into the laser cavity by non-physical-contact connectors. The "medium" level of laser coherence required for 1-Gb/sec. transmission can usually be sustained in the face of reflections from physical-contact connectors if only minor measures are taken to minimize reflections in coupling the laser output to the optic fiber. But more-serious measures must be taken to maintain enough coherence if the network employs non-physical-contact connectors. Specifically, optical isolators must be interposed between the laser and the cable.
Unfortunately, conventional optical isolators, which use polarization effects to attenuate reflected light, are quite expensive; in some cases it would be less expensive simply to rewire with physical-contact connectors (and, typically, single-mode cable) than to keep the existing non-physical-contact plant and employ an optical isolator at each transmitter.